Crystal structures, intermolecular interactions and fluorescence properties of a series of symmetrical bi-1,3,4-oxadiazole derivatives
Six crystal structures of a series of symmetrical phenyl and thienyl substituted bi-1,3,4-oxadiazole derivatives are reported here. All of these derivatives adopted a π-stacking structure and self-assembled into molecular layers as determined by single crystal X-ray diffraction. These compounds crystallized into two types of aggregations with different slip distances along the molecular axis. The parent compound BOXD, and those with fluoro-substitution in the ortho- and meta-positions of benzene (BOXD-o-F and BOXD-m-F) pack in J-aggregations which have a planar conformation, and their absorption and emission maxima show significant red-shifts. Other compounds (BOXD-p-F, BOXD-o-OCH3 and TBOXD), in which molecules show a twist conformation, pack in an arrangement with a relatively large slip distance along the molecular short axis. The maxima of BOXD-p-F and BOXD-o-OCH3 show slight red shifts in the absorption and emission spectra, while TBOXD exhibits blue shift in the emission spectrum. The Symmetry Adapted Perturbation Theory (SAPT) analysis of all of the structures with intermolecular interactions between molecules shows that the π–π stacking interaction plays a major role in the formation of the two different aggregations; however, in the aggregations where the slip distance along the molecular short axis is much longer, there would be another certain interaction such as a C–H⋯N interaction present, making a significant contribution to molecular packing. These investigations provide not only new insight and understanding into molecular packing and its driving force, but also guidance for preparing materials with excellent device performance.